Abnormality detection device for reductant addition valve

ABSTRACT

An addition valve is instructed to add a reductant (step  122 ). A cumulative specified addition amount is then determined (step  124 ). An air-fuel ratio A/Fcal is estimated in accordance with the operating status of an internal combustion engine (step  126 ). An estimated air-fuel ratio A/Fcal and air-fuel ratio sensor output A/Fs are used to determine a measured addition amount (Δt) at the current moment (step  128 ). A cumulative measured addition amount is then determined (step  130 ). The error between the measured addition amount and specified addition amount are compared against a reference value (step  134 ) to detect an abnormality in the addition valve.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an abnormality detection device for a reductant addition valve that adds a reductant to an exhaust path.

2. Background Art

A known device disclosed, for instance, in Patent Document 1 estimates, in accordance with an air-fuel ratio sensor output, the amount of reductant to be added from a reductant addition valve (hereinafter may be abbreviated to the “addition valve”) and compares the estimated reductant addition amount against a specified addition amount to judge whether the addition valve is abnormal.

[Patent Document 1] JP-A-2005-54723

[Patent Document 1] JP-A-2002-38928

The reductant addition amount is estimated in accordance with the difference between a reference value, which is an air-fuel ratio detected before a rich spike, and the air-fuel ratio detected during the rich spike. However, if the combustion air-fuel ratio changes during the rich spike, a discrepancy arises between the reference value and actual air-fuel ratio. When such a discrepancy exists, the reductant addition amount cannot accurately be estimated. It is therefore conceivable that an addition valve abnormality may not accurately be detected. It means that conventional methods achieve addition value abnormality detection in a steady state only.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances. An object of the present invention is to provide a reductant addition valve abnormality detection device that is capable of accurately detecting an abnormality in a reductant addition valve even when the combustion air-fuel ratio changes during a rich spike.

According to one aspect of the present invention, a reductant addition valve abnormality detection device comprises an addition amount designation means, first air-fuel ratio detection means, second air-fuel ratio acquisition means, addition amount measurement means, and abnormality detection means.

The addition amount designation means designates the amount of reductant to be added by the reductant addition valve that adds a reductant to upstream of a catalyst to recover the purification capability of the catalyst.

The first air-fuel ratio detection means detects a first air-fuel ratio, which is an exhaust air-fuel ratio prevailing downstream of the reductant addition valve.

The second air-fuel ratio acquisition means acquires a second air-fuel ratio, which is an exhaust air-fuel ratio of an internal combustion engine, during reductant addition by the reductant addition valve.

The addition amount measurement means measures the amount of reductant added by the reductant addition valve in accordance with the second air-fuel ratio, which is acquired by the second air-fuel ratio acquisition means, and the first air-fuel ratio, which is detected by the first air-fuel ratio detection means.

The abnormality detection means detects an abnormality in the reductant addition valve by comparing an addition amount measured by the addition amount measurement means against an addition amount designated by the addition amount designation means.

According to another aspect of the present invention, the reductant addition valve abnormality detection means further comprises a difference calculation means, and correction means.

The difference calculation means calculates the difference between the first air-fuel ratio detected by the first air-fuel ratio detection means, and the second air-fuel ratio acquired by the second air-fuel ratio acquisition means, before the reductant addition valve adds the reductant.

The correction means corrects the first air-fuel ratio or the second air-fuel ratio in accordance with the difference calculated by the difference calculation means.

According to another aspect of the present invention, the second air-fuel ratio acquisition means may include at least either second air-fuel ratio estimation means, which estimates the second air-fuel ratio in accordance with an operating state of the internal combustion engine, or second air-fuel ratio detection means, which detects the second air-fuel ratio prevailing upstream of the reductant addition valve.

Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the configuration of a system according to a first embodiment of the present invention;

FIG. 2 shows an example of a method for judging an abnormality in the addition valve 44;

FIG. 3 shows how the output from the first air-fuel ratio sensor 46 changes during a rich spike operation (this output is hereinafter referred to as the “air-fuel ratio sensor output” A/Fs);

FIG. 4 illustrates a method for measuring the amount of fuel added from the addition valve 44;

FIG. 5 shows a discrepancy between the reference value A/Fbase and actual exhaust air-fuel ratio A/Fact that arises when the combustion air-fuel ratio varies during a rich spike operation;

FIG. 6 shows the reference value A/Fbase for determining the measured addition amount in accordance with the first embodiment;

FIG. 7 shows how the first embodiment offset-corrects the air-fuel ratio A/Fcal;

FIG. 8 is a flowchart illustrating a routine that the ECU 60 executes in accordance with the first embodiment;

FIG. 9 is a flowchart illustrating the addition valve abnormality detection routine that the ECU 60 executes in accordance with the first embodiment;

FIG. 10 is a flowchart illustrating an addition valve abnormality detection routine that the ECU 60 executes in accordance with a modification of the first embodiment;

FIG. 11 is a diagram illustrating the configuration of a system according to the second embodiment of the present invention;

FIG. 12 is a flowchart illustrating a routine that the ECU 60 executes in accordance with the second embodiment; and

FIG. 13 is a flowchart illustrating the addition valve abnormality detection routine that the ECU 60 executes in accordance with the second embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the present invention will now be described with reference to the accompanying drawings. Like elements in the drawings are designated by the same reference numerals and will not be redundantly described.

First Embodiment [Description of System Configuration]

FIG. 1 is a diagram illustrating the configuration of a system according to a first embodiment of the present invention. The system shown in FIG. 1 includes an internal combustion engine 1, which is a four-cycle diesel engine (compression ignition internal combustion engine). The diesel engine 1 is mounted in a vehicle and used as its motive power source. Although the diesel engine 1 shown in FIG. 1 is of an in-line four-cylinder type, the present invention is not limited to the use of four cylinders and in-line cylinder arrangement.

A piston for each cylinder 2 of the diesel engine 1 is coupled to a crankshaft 4 via a crank mechanism. A crank angle sensor 5 is installed near the crankshaft 4 to detect the rotation angle (crank angle) of the crankshaft 4.

An injector 6, which directly injects fuel into a cylinder, is installed in each cylinder 2 of the diesel engine 1. The injector 6 for each cylinder is connected to a common rail 7. The fuel in a fuel tank (not shown) is pressurized to a predetermined fuel pressure by a supply pump 8. The pressurized fuel is stored in the common rail 7 and supplied from the common rail 7 to each injector 6. The injector 6 can inject fuel into the cylinder multiple times per cycle with arbitrary timing.

An intake port 10 of the diesel engine 1 is provided with an intake valve 12. The valve opening characteristics (valve opening timing, lift amount, and operating angle) of the intake valve 12 can be changed by a publicly known variable valve train (not shown).

The intake port 10 is connected to an intake path 18 through an intake manifold 16. An intake throttle valve 20 is installed in the middle of the intake path 18. The intake throttle valve 20 is an electronically-controlled valve whose opening is determined in accordance with an accelerator opening AA detected by an accelerator opening sensor 21. An inter-cooler 22 is installed upstream of the intake throttle valve 20. A compressor 24 a for a turbocharger 24 is installed upstream of the inter-cooler 22. A coupling shaft is used to couple the compressor 24 a to a turbine 24 b in an exhaust path 38.

An air flow meter 26 is installed upstream of the compressor 24 a to detect an intake air amount Ga. An air cleaner 28 is installed upstream of the air flow meter 26.

The employed configuration, which has been described above, uses the inter-cooler 22 to cool intake air that is compressed by the compressor 24 a of the turbocharger 24. The intake air passing through the inter-cooler 22 is distributed to the intake port 10 of each cylinder by the intake manifold 16.

An exhaust port 30 of the diesel engine 1 is provided with an exhaust valve 32. The valve opening characteristics (valve opening timing, lift amount, and operating angle) of the exhaust valve 32 can be changed by a publicly known variable valve train (not shown).

The exhaust port 30 is connected to the exhaust path 38 through an exhaust manifold 36. The turbine 24 b for the turbocharger 24 is installed in the exhaust path 38. An oxidation catalyst 40, which is a pretreatment catalyst, is installed downstream of the turbine 24 b. The oxidation catalyst 40 is capable of oxidizing HC and CO.

A NOx catalyst 42 is installed downstream of the oxidation catalyst 40. In an atmosphere where the air-fuel ratio is greater than the stoichiometric air-fuel ratio, that is, in an atmosphere leaner than the stoichiometric air-fuel ratio, the NOx catalyst 42 is capable of occluding NOx in an exhaust gas. In a rich atmosphere where the air-fuel ratio is not greater than the stoichiometric air-fuel ratio, the NOx catalyst 42 is capable of reducing and purifying occluded NOx and releasing it.

The NOx catalyst 42 may be either a catalyst capable of merely occluding and reducing NOx or a DPNR (Diesel Particulate-NOx Reduction system) or other similar catalyst capable of occluding and reducing NOx and collecting soot in the exhaust gas. The NOx catalyst 42 may also be a catalyst capable of occluding and reducing NOx and having a function other than the soot collection function. The oxidation catalyst 40 and NOx catalyst 42 may be housed in a single container.

An exhaust fuel addition valve (hereinafter abbreviated to the “addition valve”) 44 is installed between the turbine 24 b and oxidation catalyst 40 to add fuel to the exhaust gas. The added gas serves as a reductant. The addition valve 44 communicates with the supply pump 8 through a fuel supply pipe (not shown).

A first air-fuel ratio sensor 46 is installed between the oxidation catalyst 40 and NOx catalyst 42. A second air-fuel ratio sensor 48 is installed downstream of the NOx catalyst 42. These air-fuel ratio sensors 46, 48 are configured to detect exhaust air-fuel ratios prevailing at their positions, respectively.

One end of an external EGR path 52 is connected to a section near the intake manifold 16 in the intake path 18. The other end of the external EGR path 52 is connected to a section near the exhaust manifold 36 in the exhaust path 38. The system uses the external EGR path 52 to provide external EGR (Exhaust Gas Recirculation). More specifically, the system can flow part of the exhaust gas (burned gas) back to the intake path 18 through the external EGR path 52.

An EGR cooler 54 is installed in the middle of the external EGR path 52 to cool an external EGR gas. An EGR valve 56 is installed downstream of the EGR cooler 54 in the external EGR path 52. The amount of exhaust gas flowing in the external EGR path 52 (that is, the external EGR amount or external EGR rate) increases with an increase in the opening of the EGR valve 56.

The system according to the first embodiment also includes an ECU (Electronic Control Unit) 60, which serves as a controller. The output end of the ECU 60 is connected, for instance, to the injector 6, supply pump 8, intake throttle valve 20, addition valve 44, and EGR valve 56. The input end of the ECU 60 is connected, for instance, to the crank angle sensor 5, accelerator opening sensor 21, air flow meter 26, first air-fuel ratio sensor 46, and second air-fuel ratio sensor 48.

The ECU 60 calculates an engine speed NE in accordance with an output from the crank angle sensor 5. The ECU 60 also calculates an engine load KL in accordance, for instance, with the accelerator opening AA. The ECU 60 calculates the amount of fuel injection from the injector 6 (in-cylinder fuel injection amount) Q in accordance with the engine load KL. In accordance with signals from various sensors and with a predetermined program, the ECU 60 activates various actuators to control the operating status of the diesel engine 1.

Features of First Embodiment

When the amount of NOx occluded by the NOx catalyst 42 in the system exceeds a predetermined value, a so-called rich spike operation is performed to reduce and release NOx. More specifically, the addition valve 44 adds fuel that serves as a reductant. The added reductant performs a purification capability recovery process (regeneration process) on the NOx catalyst 42.

Meanwhile, it may be demanded that an abnormality in addition amount (injection amount) control over the addition valve 44 be accurately detected.

FIG. 2 shows an example of a method for judging an abnormality in the addition valve 44. In FIG. 2, the horizontal axis represents a specified addition amount, which is indicated from the ECU 60 to the addition valve 44, whereas the vertical axis represents a measured addition amount, which is measured (estimated) by a method described later.

As shown in FIG. 2, an abnormality in the addition valve 44 can be detected by comparing the measured addition amount against the specified addition amount. If the ratio R of the measured addition amount to the specified addition amount is 1, that is, if the specified amount of reductant is added by the addition valve 44, it goes without saying that the addition valve 44 is normal. However, it is necessary to consider variations and aging of the addition valve 44. Thus, the addition valve 44 can be judged to be normal when the ratio R is within a reference range shown in FIG. 2, and abnormal (defective) when the ratio R is outside the reference range. More specifically, if the specified addition amount is equal to a predetermined amount A and the error (absolute value) between the specified addition amount A and measured addition amount is not greater than a reference value Dth, the addition valve 44 can be judged to be normal; however, if the error is greater than the reference value Dth, the addition valve 44 can be judged to be abnormal (defective). The reference range or reference value Dth can be predefined for each vehicle type.

FIG. 3 shows how the output from the first air-fuel ratio sensor 46 changes during a rich spike operation (this output is hereinafter referred to as the “air-fuel ratio sensor output” A/Fs).

While no rich spike operation is performed, that is, no fuel is added from the addition valve 44, the air-fuel ratio sensor output A/Fs looks like an output A/F_A indicated by a one-dot chain line in FIG. 3. During rich spike time t, on the other hand, the addition valve 44 adds fuel; therefore, the air-fuel ratio sensor output A/Fs changes to an output A/F_B indicated by a solid line in FIG. 3.

Although details will be given later, the amount of fuel added from the addition valve 44 can be measured (estimated) in accordance with the difference between a reference value A/Fbase, which is the air-fuel ratio sensor output A/F_A generated while no rich spike operation is performed, and the air-fuel ratio sensor output A/F_B generated while a rich spike operation is performed. In other words, the aforementioned measured addition amount can be determined in accordance with the air-fuel ratio sensor outputs A/Fs (A/F_A, A/F_B).

FIG. 4 illustrates a method for measuring the amount of fuel added from the addition valve 44. In FIG. 4, the symbol “A/Fs” denotes an air-fuel ratio sensor output. The symbol “A/Fs (t0)” denotes an air-fuel ratio sensor output that is generated while no rich spike operation is performed, or more specifically, at time t0, which is immediately before the start of a rich spike operation.

The air-fuel ratio sensor output A/Fs (t0) at time t0 can be expressed by Equation (1) below. In Equation (1), “Ga” is an intake air amount and “Q” is an in-cylinder fuel injection amount.

A/Fs(t0)=Ga/Q   (1)

Equation (2) below is obtained by modifying Equation (1) above.

Q=Ga/(A/F(t0))   (2)

The air-fuel ratio sensor output A/Fs generated during a rich spike operation can be expressed by Equation (3) below. In Equation (3), “Qex” is the amount of fuel addition from the addition valve 44.

A/Fs=Ga/(Q+Qex)   (3)

Equation (4) below is obtained by modifying Equation (3) above.

Q+Qex=Ga/(A/Fs)   (4)

Equation (5) below is obtained by subtracting Equation (2) from Equation (4). Further, Equation (6) below is obtained by modifying Equation (5).

$\begin{matrix} {{Qex} = {{Ga} \times \left\{ {{1/\left( {A/{Fs}} \right)} - {1/\left( {A/{{Fs}\left( {t\; 0} \right)}} \right)}} \right\}}} & (5) \\ {= {{Ga} \times {\left\{ {\left( {A/{{Fs}\left( {t\; 0} \right)}} \right) - \left( {A/{Fs}} \right)} \right\}/\left( {{A/{{Fs}\left( {t\; 0} \right)}}/\left( {A/{Fs}} \right)} \right.}}} & (6) \end{matrix}$

The air-fuel ratio sensor output A/Fs (t0) in Equation (6) above can be acquired at time t0, which is immediately before the start of a rich spike operation. Therefore, when the intake air amount Ga and air-fuel ratio sensor output A/Fs are known, the amount of fuel addition at a certain point of time during a rich spike operation can be calculated from Equation (6). The amount of fuel added from the addition valve 44 during a rich spike operation can be measured by adding up the amounts of fuel addition during the rich spike time t shown in FIG. 4. In other words, the measured addition amount is obtained.

The above method uses the air-fuel ratio sensor output A/Fs (t0) generated immediately before the start of a rich spike operation as the reference value A/Fbase and determines the measured addition amount in accordance with the difference between the reference value A/Fbase and air-fuel ratio sensor output A/Fs (see FIGS. 3 and 4).

However, the combustion air-fuel ratio may vary during a rich spike operation (due, for instance, to a transient state) as shown in FIG. 5. If, in such an instance, the air-fuel ratio sensor output A/Fs (t0) generated immediately before the start of a rich spike operation is used as the reference value A/Fbase as mentioned above, a discrepancy arises between the reference value A/Fbase and actual exhaust air-fuel ratio A/Fact (the air-fuel ratio prevailing upstream of the addition valve 44). FIG. 5 shows a discrepancy between the reference value A/Fbase and actual exhaust air-fuel ratio A/Fact that arises when the combustion air-fuel ratio varies during a rich spike operation. In this instance, the accuracy of calculating the measured addition amount decreases because an inaccurate reference value A/Fbase is used to determine the measured addition amount.

As such being the case, the first embodiment estimates the exhaust air-fuel ratio A/Fcal in accordance with the operating status (Ga, Ne, and Q) of the internal combustion engine 1 that prevails during a rich spike operation. In the case, for instance, of the in-line four-cylinder diesel engine shown in FIG. 1, which injects fuel two times per revolution, the above-mentioned exhaust air-fuel ratio A/Fcal can be estimated from Equation (7) below when the density [g/cm³] of light oil is 0.833. “q” is a quantity of each fuel injection that relates to Q here.

A/Fcal=Ga/{(Ne/60)×2×q×0.833}  (7)

FIG. 6 shows the reference value A/Fbase for determining the measured addition amount in accordance with the first embodiment.

As shown in FIG. 6, the first embodiment uses the exhaust air-fuel ratio A/Fcal estimated from Equation (7) as the reference value A/Fbase. More specifically, the above estimated exhaust air-fuel ratio A/Fcal is used instead of the air-fuel ratio sensor output A/Fs (t0) in Equation (6) to calculate the fuel addition amount Qex. The exhaust air-fuel ratio A/Fcal follows changes in the combustion air-fuel ratio. Therefore, the reference value A/Fbase for determining the measured addition amount can follow combustion air-fuel ratio changes during a rich spike operation. Consequently, the measured addition amount can be accurately determined even when the combustion air-fuel ratio changes during a rich spike operation as if a transient state prevails.

Further, the air flow meter 26, the injector 6, and the first air-fuel ratio sensor 46 vary respectively. Therefore, even when these elements operate normally, the above estimated exhaust air-fuel ratio A/Fcal may disagree with the air-fuel ratio sensor output A/Fs as indicated in FIG. 7. In such an instance, the measured addition amount cannot accurately be determined either.

As such being the case, the first embodiment determines the difference ΔA/F between the exhaust air-fuel ratio A/Fcal and exhaust air-fuel ratio A/Fs before a rich spike operation as shown in FIG. 7. Then, during the rich spike operation, the first embodiment offset-corrects the exhaust air-fuel ratio A/Fcal by the amount of the difference ΔA/F. FIG. 7 shows how the first embodiment offset-corrects the air-fuel ratio A/Fcal. The offset correction shown in FIG. 7 can absorb the influence of variations in the air flow meter 26, injector 6, and first air-fuel ratio sensor 46. Since this makes it possible to eliminate the difference between the air-fuel ratio A/Fcal and air-fuel ratio sensor output A/Fs, which exists when no reductant is added, the measured addition amount can be accurately determined.

Details of Process Performed by First Embodiment

FIG. 8 is a flowchart illustrating a routine that the ECU 60 executes in accordance with the first embodiment. The routine shown in FIG. 8 is started at predetermined time intervals.

First of all, the routine shown in FIG. 8 performs step 100 to judge whether an addition valve abnormality detection request flag is ON. The addition valve abnormality detection request flag turns ON when the vehicle has traveled over a predetermined distance or for a predetermined period of time after last abnormality detection or when a predetermined value is exceeded by an addition amount specified for the addition valve 44 after last abnormality detection.

If the judgment result obtained in step 100 indicates that the addition valve abnormality detection request flag is ON, step 102 is performed to judge whether an offset calculation completion flag is ON. The offset calculation completion flag turns ON when the difference (offset) between the air-fuel ratio sensor output A/Fs and estimated air-fuel ratio A/Fcal is completely calculated before addition valve abnormality detection.

If the judgment result obtained in step 102 indicates that the offset calculation completion flag is OFF, the flow proceeds to step 104. In step 104, the intake air amount Ga, in-cylinder fuel injection amount Q, and engine speed Ne are used to calculate the air-fuel ratio A/Fcal from Equation (7). In other words, step 104 is performed to estimate the exhaust air-fuel ratio A/Fcal of the internal combustion engine 1 in accordance with the operating status (Ga, Q, and Ne) of the internal combustion engine 1.

Subsequently, step 106 is performed so that a numerical value obtained by subtracting the air-fuel ratio sensor output A/Fs (the air-fuel ratio detected by the air-fuel ratio sensor 46) from the air-fuel ratio A/Fcal estimated in step 104 is set as the air-fuel ratio difference (offset) ΔA/F. More specifically, the difference ΔA/F between the estimated air-fuel ratio A/Fcal and air-fuel ratio sensor output A/Fs is determined. This difference ΔA/F is used to offset-correct the estimated air-fuel ratio A/Fcal in step 126, which will be described later with reference to FIG. 9.

Next, step 108 is performed to turn ON the offset calculation completion flag. Subsequently, step 110 is performed to set the specified addition amount to zero (0). Step 112 is then performed to set the measured addition amount to zero (0). In other words, steps 110 and 112 are performed to reset the specified addition amount and measured addition amount.

Upon completion of step 112, the routine temporarily terminates. When the routine is restarted later, step 100 is performed. If the judgment result obtained in step 100 indicates that the addition valve abnormality detection request flag is OFF, that is, the addition valve 44 is not to be checked for an abnormality, step 114 is performed to turn OFF the offset calculation completion flag.

If, on the other hand, the judgment result obtained in step 100 indicates that the addition valve abnormality detection request flag is ON, step 102 is performed. The offset calculation completion flag was turned ON in step 108 of the last-executed routine. Therefore, the query in step 102 is answered “Yes.” This starts an addition valve abnormality detection routine shown in FIG. 9. FIG. 9 is a flowchart illustrating the addition valve abnormality detection routine that the ECU 60 executes in accordance with the first embodiment.

First of all, the routine shown in FIG. 9 performs step 120 to judge whether the specified addition amount is not smaller than a predetermined value. The predetermined value denotes a fuel amount that permits addition valve abnormality detection, that is, a specified addition amount that permits accurate comparison between the specified addition amount and measured addition amount. An appropriate value can be predetermined for each vehicle type. If the judgment result obtained in step 120 indicates that the specified addition amount is smaller than the predetermined value, step 122 is performed to instruct the addition valve 44 to add fuel that serves as a reductant. Subsequently, step 124 is performed so that a numerical value obtained by adding the currently specified addition amount, which is specified in step 122, to the previously specified addition amount is set as the specified addition amount. In step 124, the specified addition amounts obtained after a reset in step 110 are added up.

Next, step 126 is performed to estimate the air-fuel ratio A/Fcal, which serves as the reference value A/Fbase (see FIGS. 6 and 7), in accordance with the operating status (Ga, Q, and Ne) of the internal combustion engine 1. In step 126, the intake air amount Ga, in-cylinder fuel injection amount Q, and engine speed Ne are used to calculate the air-fuel ratio A/Fcal from Equation (7). Further, in step 126, a numerical value obtained by subtracting the offset ΔA/F calculated in step 106 (FIG. 8) from the exhaust air-fuel ratio A/Fcal of the internal combustion engine 1, which is calculated from Equation (7), is set as the air-fuel ratio A/Fcal. In other words, the estimated air-fuel ratio A/Fcal is offset-corrected.

Next, step 128 is performed to calculate the measured addition amount (Δt). The measured addition amount (Δt) is a measured addition amount at the current moment Δt. In step 128, a numerical value obtained by subtracting a numerical value (Ga/A/Fcal), which is obtained by dividing the intake air amount Ga by the air-fuel ratio A/Fcal offset-corrected in step 126, from a numerical value (Ga/A/Fs), which is obtained by dividing the intake air amount Ga by the air-fuel ratio sensor output A/Fs, is set as the measured addition amount (Δt).

Next, step 130 is performed so that a numerical value obtained by adding the measured addition amount (Δt), which is calculated in step 128, to the previously determined measured addition amount is set as the measured addition amount. In step 130, the measured addition amounts obtained after a reset in step 112 are added up. Upon completion of step 130, the routine temporarily terminates.

When the routine is started again, the specified addition amounts and measured addition amounts are added up by sequentially performing steps 122 to 130 until the cumulative specified addition amount reaches the predetermined value.

Meanwhile, when a supplied fuel amount is not smaller than a predetermined value, the query in step 120 is answered “Yes.” Next, step 132 is performed to determine an estimated error by subtracting the specified addition amount from the measured addition amount. Step 134 is then performed to judge whether the estimated error determined in step 132 is greater than a reference value. The reference value represents a permissible estimated error for judging that the addition valve 44 is normal. This reference value is an abnormality judgment value for the addition valve 44. The reference value Dth shown in FIG. 2 is an example of this reference value.

If the judgment result obtained in step 134 indicates that the estimated error is greater than the reference value, it is concluded that the addition valve 44 is abnormal (defective). In this instance, step 136 is performed to turn ON an addition valve abnormality flag. If, on the other hand, the judgment result obtained in step 134 does not indicate that the estimated error is greater than the reference value, it is concluded that the addition valve 44 is normal. In this instance, step 138 is performed to turn OFF the addition valve abnormality flag. Upon completion of step 136 or 138, step 140 is performed to turn OFF the addition valve abnormality detection request flag.

As described above, the routine shown in FIGS. 8 and 9 determines the measured addition amount in accordance with the air-fuel ratio A/Fcal estimated from the operating status (Ga, Q, and Ne) of the internal combustion engine 1 during a rich spike operation and with the air-fuel ratio sensor output A/Fs instead of the air-fuel ratio sensor output A/Fs (t0) generated immediately before the rich spike operation (at time t0). The air-fuel ratio A/Fcal can follow combustion air-fuel ratio changes in the internal combustion engine 1 during a rich spike operation. Therefore, the measured addition amount can be accurately determined even when the combustion air-fuel ratio changes during a rich spike operation due, for instance, to a transient state. Consequently, an abnormality in the addition valve 44 can be accurately detected even when the combustion air-fuel ratio changes.

Further, an abnormality detection sequence can be performed with increased frequency because abnormality detection can be achieved even when the combustion air-fuel ratio changes. Consequently, injection amount control over the addition valve 44 can be frequently monitored.

Furthermore, the routine calculates the difference ΔA/F between the estimated air-fuel ratio A/Fcal and air-fuel ratio sensor output A/Fs before a rich spike operation. During the rich spike operation, the routine offset-corrects the air-fuel ratio A/Fcal by the amount of the calculated difference ΔA/F. This eliminates the air-fuel ratio difference ΔA/F that exists when no rich spike operation is performed. As a result, the measured addition amount can be calculated with increased accuracy.

Although it is assumed that the first embodiment offset-corrects the estimated air-fuel ratio A/Fcal, an alternative would be to offset-correct the air-fuel ratio sensor output A/Fs by the amount of the difference ΔA/F. FIG. 10 is a flowchart illustrating an addition valve abnormality detection routine that the ECU 60 executes in accordance with a modification of the first embodiment. The routine shown in FIG. 10 performs step 127 instead of step 126 of the routine shown in FIG. 9. In step 127, the air-fuel ratio sensor output A/Fs is offset-corrected by the amount of the air-fuel ratio difference ΔA/F calculated in step 106 in FIG. 8. This modified embodiment can provide the same advantages as the first embodiment because the former can eliminate the air-fuel ratio difference ΔA/F that exists when no rich spike operation is performed.

It is also assumed that the first embodiment uses the output A/Fs of the first air-fuel ratio sensor 46 to determine the measured addition amount. Alternatively, however, the output of the second air-fuel ratio sensor 48 may be used to determine the measured addition amount. It should be noted, however, that the use of the output of the first air-fuel ratio sensor 46 determines the measured addition amount with higher accuracy than the use of the output of the second air-fuel ratio sensor 48.

Further, the first embodiment assumes that the air-fuel ratio remains unchanged before and after the oxidation catalyst 40, and uses the air-fuel ratio sensor output A/Fs without correcting it. Alternatively, however, a publicly known catalyst model may be used to correct the air-fuel ratio sensor output A/Fs.

In the first embodiment, the addition valve 44 is positioned between the turbine 24 b and oxidation catalyst 40. Alternatively, however, the addition valve 44 may be positioned upstream of the turbine 24 b (e.g., in the exhaust port 30 or exhaust manifold 36). The use of this alternative also provides the same advantages as the first embodiment.

Furthermore, the first embodiment checks for an abnormality in the addition valve 44 by comparing the estimated error against the reference value. However, an alternative would be to check for such an abnormality by comparing the ratio R of the measured addition amount to the specified addition amount against a reference value.

In the first embodiment, the NOx catalyst 42 corresponds to the “catalyst” according to the first aspect of the present invention; the addition valve 44 corresponds to the “reductant addition valve” according to the first aspect of the present invention; and the first air-fuel ratio sensor 46 corresponds to the “first air-fuel ratio detection device” according to the first aspect of the present invention. Further, in the first embodiment, the “addition amount designation device” according to the first aspect of the present invention is implemented when the ECU 60 performs step 122; the “second air-fuel ratio acquisition device” according to the first aspect of the present invention and the “second air-fuel ratio estimation device” according to the third aspect of the present invention are implemented when the ECU 60 performs step 126; the “addition amount measurement device” according to the first aspect of the present invention is implemented when the ECU 60 performs step 128; and the “abnormality detection device” according to the first aspect of the present invention is implemented when the ECU 60 performs steps 132, 134, 136, and 138. In the first embodiment or its modification, the “difference calculation device” according to the second aspect of the present invention is implemented when the ECU 60 performs step 106; and the “correction device” according to the second aspect of the present invention is implemented when the ECU 60 performs step 126 or 127.

Second Embodiment

A second embodiment of the present invention will now be described with reference to FIG. 11.

FIG. 11 is a diagram illustrating the configuration of a system according to the second embodiment of the present invention. The system shown in FIG. 11 differs from the system shown in FIG. 1 in that the former additional includes an air-fuel ratio sensor 45, which is positioned upstream of the addition valve 44. This air-fuel ratio sensor 45 can detect the exhaust air-fuel ratio of the internal combustion engine 1 even while fuel is being added by the addition valve 44. The other elements of the system shown in FIG. 11 are the same as the counterparts of the system shown in FIG. 1 and will not be redundantly described.

Features of Second Embodiment

As indicated in FIG. 4, the output A/Fs generated from the air-fuel ratio sensor 46 during a rich spike operation differs from the exhaust air-fuel ratio of the internal combustion engine 1. The reason is that the air-fuel ratio sensor 46 is positioned downstream of the addition valve 44. During a rich spike operation, therefore, the method shown in FIG. 4 uses the air-fuel ratio sensor output A/Fs (t0) generated immediately before the rich spike operation as the reference value A/Fbase for determining the measured addition amount.

However, the reference value A/Fbase cannot follow combustion air-fuel ratio changes during a rich spike operation as described in conjunction with the first embodiment. As such being the case, the first embodiment uses the air-fuel ratio A/Fcal estimated in accordance with the operating status (Ga, Q, and Ne) of the internal combustion engine 1 as the reference value A/Fbase for determining the measured addition amount. In other words, the first embodiment determines the measured addition amount in accordance with the difference between the air-fuel ratio A/Fcal and air-fuel ratio sensor output A/Fs.

Meanwhile, the system according to the second embodiment, which is shown in FIG. 11, includes the air-fuel ratio sensor 45 that is positioned upstream of the addition valve 44. Therefore, the air-fuel ratio sensor 45 can be used to detect the exhaust air-fuel ratio of the internal combustion engine 1 even while fuel is being added from the addition valve 44. In other words, the output A/Fsu generated from the air-fuel ratio sensor 45 can follow combustion air-fuel ratio changes during a rich spike operation.

As such being the case, the second embodiment uses the air-fuel ratio sensor output A/Fsu as the reference value A/Fbase for determining the measured addition amount. More specifically, the measured addition amount is determined in accordance with the difference between the air-fuel ratio sensor output A/Fsu and air-fuel ratio sensor output A/Fs. Consequently, the measured addition amount can be accurately determined even when the combustion air-fuel ratio changes during a rich spike operation as if a transient state prevails.

Further, the air-fuel ratio sensor 45 and air-fuel ratio sensor 46 vary respectively. Therefore, even when both of these two air-fuel ratio sensors 45, 46 operate normally, the air-fuel sensor output A/Fsu and air-fuel ratio sensor output A/Fs may not agree with each other while no rich spike operation is being performed.

Under the above circumstances, the second embodiment determines the difference ΔA/Fs between the above two air-fuel ratio sensor outputs A/Fsu, A/Fs before a rich spike operation. In addition, the second embodiment offset-corrects either of the above two air-fuel ratio sensor outputs by the amount of the difference ΔA/Fs. This offset correction absorbs the influence of variations in the air-fuel ratio sensors 45, 46. Consequently, the difference ΔA/Fs between the two air-fuel ratio sensor outputs A/Fsu, A/Fs, which exists when no reductant is added, can be eliminated. As a result, the measured addition amount can be accurately determined.

Details of Process Performed by Second Embodiment

FIG. 12 is a flowchart illustrating a routine that the ECU 60 executes in accordance with the second embodiment. The routine shown in FIG. 12 is started at predetermined time intervals. The routine shown in FIG. 12 performs step 142 instead of steps 104 and 106 of the routine shown in FIG. 8. The process performed in step 142 will be mainly described below.

If the judgment result obtained in step 102 does not indicate that the offset calculation completion flag is ON, the routine shown in FIG. 12 proceeds to step 142. In step 142, a numerical value obtained by subtracting the output A/Fs of the air-fuel ratio sensor 46 from the output A/Fsu of the air-fuel ratio sensor 45 is set as the air-fuel ratio difference (offset) ΔA/Fs. The difference ΔA/Fs is used to offset-correct the air-fuel ratio sensor output A/Fsu in step 144, which will be described later with reference to FIG. 13. After completion of step 142, step 108 is performed to turn ON the offset calculation completion flag as is the case with the routine shown in FIG. 8.

If, on the other hand, the judgment result obtained in step 102 indicates that the offset calculation completion flag is ON, an addition valve abnormality detection routine shown in FIG. 13 is started. FIG. 13 is a flowchart illustrating the addition valve abnormality detection routine that the ECU 60 executes in accordance with the second embodiment. The routine shown in FIG. 13 performs steps 144, 146, and 148 instead of steps 126, 128, and 130 of the routine shown in FIG. 9. The process performed in steps 144, 146, and 148 will be mainly described below.

After the specified addition amounts are added up in step 124, the routine shown in FIG. 13 proceeds to step 144. In step 144, a numerical value obtained by subtracting the offset ΔA/Fs calculated in step 142 (FIG. 12) from the air-fuel ratio sensor output A/Fsu is set as the air-fuel ratio sensor output A/Fsu. In other words, the air-fuel ratio sensor output A/Fsu is offset-corrected.

Subsequently, step 146 is performed to calculate the measured addition amount (Δt). The measured addition amount (Δt) is a measured addition amount at the current moment Δt. In step 146, a numerical value obtained by subtracting a numerical value (Ga/A/Fs), which is obtained by dividing the intake air amount Ga by the air-fuel ratio sensor output A/Fs, from a numerical value (Ga/A/Fsu), which is obtained by dividing the intake air amount Ga by the air-fuel ratio sensor output A/Fsu, is set as the measured addition amount (Δt).

Next, step 148 is performed so that a numerical value obtained by adding the measured addition amount (Δt), which is calculated in step 146, to the previously determined measured addition amount is set as the measured addition amount. In step 148, the measured addition amounts obtained after a reset in step 112 (FIG. 12) are added up. Upon completion of step 148, the routine temporarily terminates.

When the routine is started again, the specified addition amounts and measured addition amounts are added up by sequentially performing steps 122, 124, and 144 to 148 until the cumulative specified addition amount reaches the predetermined value.

However, when the specified addition amount is not smaller than the predetermined value, the query in step 120 is answered “Yes” as is the case with the routine shown in FIG. 9. Subsequently, steps 132 to 140 are performed as is the case with the routine shown in FIG. 9.

As described above, the routine shown in FIGS. 12 and 13 determines the measured addition amount in accordance with the air-fuel ratio sensor output A/Fsu and air-fuel ratio sensor output A/Fs generated during a rich spike operation instead of the air-fuel ratio sensor output A/Fs (t0) generated immediately before the rich spike operation (at time t0). This air-fuel ratio sensor output A/Fsu can follow combustion air-fuel ratio changes of the internal combustion engine 1 during a rich spike operation. Therefore, the measured addition amount can be accurately determined even when the combustion air-fuel ratio changes during a rich spike operation as if a transient state prevails. Consequently, an abnormality in the addition valve 44 can be accurately detected even when the combustion air-fuel ratio changes.

Further, the routine calculates the difference ΔA/Fs between the air-fuel ratio sensor output A/Fsu and air-fuel ratio sensor output A/Fs before a rich spike operation. In addition, the routine offset-corrects the air-fuel ratio sensor output A/Fsu by the amount of the calculated difference ΔA/Fs during the rich spike operation. This eliminates the air-fuel ratio sensor output difference ΔA/Fs that exists when no rich spike operation is performed. As a result, the measured addition amount can be calculated with increased accuracy.

It is assumed that the second embodiment offset-corrects the air-fuel ratio sensor output A/Fsu by the amount of the difference ΔA/Fs. Alternatively, however, the air-fuel ratio sensor output A/Fs may be offset-corrected by the amount of the difference ΔA/Fs. The use of such an alternative also makes it possible to eliminate the air-fuel ratio sensor output difference ΔA/Fs that exists when no rich spike operation is performed.

Further, the second embodiment, which has been described above, assumes that the air-fuel ratio sensor output A/Fsu is greater than the air-fuel ratio sensor output A/Fs by ΔA/Fs. However, it is obvious that the present invention can also be applied to a case where the air-fuel ratio sensor output A/Fsu is smaller than the air-fuel ratio sensor output A/Fs by ΔA/Fs. In such a case, step 144 should be performed so that a numerical value obtained by adding the difference ΔA/Fs to the air-fuel ratio sensor output A/Fsu is set as the air-fuel ratio sensor output A/Fsu.

In the second embodiment, the air-fuel ratio sensor 45 corresponds to the “second air-fuel ratio acquisition device” according to the first aspect of the present invention and the “second air-fuel ratio detection device” according to the third aspect of the present invention. Further, in the second embodiment, the “difference calculation device” according to the second aspect of the present invention is implemented when the ECU 60 performs step 142; and the “correction device” according to the second aspect of the present invention is implemented when the ECU 60 performs step 144. 

1. A reductant addition valve abnormality detection device for detecting an abnormality in a reductant addition valve that adds a reductant to upstream of a catalyst to recover the purification capability of the catalyst, the reductant addition valve abnormality detection device comprising: addition amount designation means for designating the amount of reductant to be added by said reductant addition valve; first air-fuel ratio detection means for detecting a first air-fuel ratio, which is an exhaust air-fuel ratio prevailing downstream of said reductant addition valve; second air-fuel ratio acquisition means for acquiring a second air-fuel ratio, which is an exhaust air-fuel ratio of an internal combustion engine, during reductant addition by said reductant addition valve; addition amount measurement means for measuring the amount of reductant added by said reductant addition valve in accordance with the second air-fuel ratio, which is acquired by said second air-fuel ratio acquisition means, and the first air-fuel ratio, which is detected by said first air-fuel ratio detection means; and abnormality detection means for detecting an abnormality in said reductant addition valve by comparing an addition amount measured by said addition amount measurement means against an addition amount designated by said addition amount designation means.
 2. The reductant addition valve abnormality detection device according to claim 1, further comprising: difference calculation means for calculating the difference between the first air-fuel ratio detected by said first air-fuel ratio detection means, and the second air-fuel ratio acquired by said second air-fuel ratio acquisition means, before said reductant addition valve adds the reductant; and correction means for correcting said first air-fuel ratio or said second air-fuel ratio in accordance with the difference calculated by said difference calculation means.
 3. The reductant addition valve abnormality detection device according to claim 1, wherein said second air-fuel ratio acquisition means includes at least either second air-fuel ratio estimation means, which estimates said second air-fuel ratio in accordance with an operating state of said internal combustion engine, or second air-fuel ratio detection means, which detects said second air-fuel ratio prevailing upstream of said reductant addition valve.
 4. A reductant addition valve abnormality detection device for detecting an abnormality in a reductant addition valve that adds a reductant to upstream of a catalyst to recover the purification capability of the catalyst, the reductant addition valve abnormality detection device comprising: addition amount designation device for designating the amount of reductant to be added by said reductant addition valve; first air-fuel ratio detection device for detecting a first air-fuel ratio, which is an exhaust air-fuel ratio prevailing downstream of said reductant addition valve; second air-fuel ratio acquisition device for acquiring a second air-fuel ratio, which is an exhaust air-fuel ratio of an internal combustion engine, during reductant addition by said reductant addition valve; addition amount measurement device for measuring the amount of reductant added by said reductant addition valve in accordance with the second air-fuel ratio, which is acquired by said second air-fuel ratio acquisition device, and the first air-fuel ratio, which is detected by said first air-fuel ratio detection device; and abnormality detection device for detecting an abnormality in said reductant addition valve by comparing an addition amount measured by said addition amount measurement device against an addition amount designated by said addition amount designation device. 